FDMA digital radio technologies such as dPMR and NXDN may not be as well known as DMR, but they have plenty to offer, as Sam Fenwick discovers

Before we get into the meat of this piece, a bit of technical explanation is needed. Frequency Divided Multiple Access (FDMA) allows the use of two channels within a 12.5kHz chunk of spectrum. As its name suggests – the two channels are separated by frequency. In contrast, DMR uses Time Divided Multiple Access (TDMA) technology to divide the 12.5kHz channel into two time-slots, giving the same effective capacity (see the diagram below).

There are two main technologies on the market that use FDMA: dPMR and NXDN. dPMR is an open standard developed by ETSI as a low-cost, low-complexity technology. NXDN is an ITU standard and has been implemented by Kenwood as NEXEDGE and by Icom as IDAS. It can operate in 6.25kHz or 12.5kHz channels, while dPMR uses only 6.25kHz.

Time and the unceasing efforts of two-way manufacturers’ R&D departments have eroded a lot of the differences between FDMA technologies and TDMA (DMR) from an end-user’s perspective. “Both dPMR and DMR air interface protocols have developed significantly since introduction. DMR now has options to provide simple and inexpensive systems, while dPMR can offer multi-site and wide area systems-based solutions including Mode 3 trunking,” says Mike Atkins, director at Kenwood and chair of the dPMR Association’s marketing group.

The lines have been further blurred by the fact that manufacturers can offer radios that can support multiple air interfaces; for example, Kenwood offers two-way radios that support NXDN, DMR and analogue. In addition, Kenwood’s (mid-tier) and Icom’s (currently available) NXDN equipment are upgradable to dPMR.

Pete Hizzey, chairman of the dPMR Association, explains that this is aided by the fact that while dPMR and NXDN are separate air interfaces, they run off the same hardware. “You could think of an IDAS radio as being like a PC and you can either load it with NXDN software or dPMR software, but the hardware is the same,” he says.

However, subtle differences remain. For example, FDMA technologies have slighter better range than their DMR counterparts, though this results in slightly less battery life – but in Hizzey’s view this is a worthwhile trade off.

“There’s a slight difference, because TDMA only transmits for half of the time [of FDMA], but with dPMR you’re operating with narrower bandwidth, which gives higher sensitivity, so if you have the option, you can turn the power down for equivalent performance. That would give you increased battery life, but users are typically more concerned with having the [maximum] range [possible], so they’re not concerned about backing off the power a bit to get quality of battery life.

“Battery technology these days is so good that we can give 14 hours’ performance on a FDMA portable.”

Hizzey adds that the difference between dPMR and DMR is that the former is “always ad-hoc. You don’t have to have a network in control to make sure that everything works properly, because the fundamental problem with TDMA is that some entity has to control the timing of all the mobiles within the system otherwise you get interference.”

Atkins confirms that over-the-air programming (OTAP) is available with dPMR, and that while it is often useful when providing communications for events, it is crucial for public access systems; for example, if a customer hasn’t paid their bills and needs to be disconnected from the service, or if they need to expand their radio fleet.

“dPMR has proved itself in the UK in the licence-free dPMR446 format and in countries where 6.25kHz channel allocations are made. For example, we have dPMR trunking systems in operation outside the UK, where a user specifically needed to create a trunking system, and the only way that the authority could give them the right frequencies was by allocating a block of 6.25kHz channels, so dPMR was perfect. It really depends on the exact needs of each user,” says Atkins.

Ian Lockyer, Icom UK’s marketing manager, says both conventional and trunked dPMR systems are in use in the UK, Spain, Germany and many other European countries. He adds that dPMR users span a wide variety of sectors including prisons, humanitarian organisations (the UN is one of the biggest adopters of dPMR), security companies, nuclear facilities, wharfs, transport and municipal councils.

Regarding licence-free PMR (dPMR446), Atkins adds that in such applications, FDMA’s nominal range advantage over analogue and TDMA technologies and double the available channels can offer real benefits: “If you’re a professional peer-to-peer user then the range down to the last third of a mile could be crucial, while if users find themselves in congested areas like city centres or during large events, the availability of additional channels is a real benefit.”

Hizzey says that while certain DMR features are presented as giving it an advantage over dPMR, “some of these are not something that is of use to people, things like having full duplex communication or even to have emergency reverse signalling. These are not aspects that most people are looking for with a two-way radio system.”

He notes that most radio licences are priced by spectrum, meaning that “if you’re only using a 6.25kHz channel, the odds are that you can get a cheaper licence than if you want to have a whole 12.5kHz channel”, as would be the case with DMR, “unless you can find somebody else who can share the other timeslot, but that would be hugely unworkable”.

I ask Atkins why, if there is so much similarity between DMR and dPMR from an end-user’s perspective, Kenwood continues to sell both technologies.

“It’s because we want to be able to supply the best system for the user,” he explains. “When a requirement is reviewed, it’s obvious which [technology] is best for [it]. Being able to provide the right system for each application is the goal.

“Because DMR has more manufacturers and hence more marketing power, a lot of users just take DMR as the default option (much as IT buyers did with IBM in the past), so, speaking as Kenwood, we often have customers that come to us saying ‘we want DMR’, and with many of those, we’ll offer them DMR where it’s appropriate.

“However, where it’s less clear-cut or where dPMR is more suitable, that’s what we’d recommend – it all depends on a multitude of factors, such as the coverage area, the size of the system, the features and the facilities they want.

“NXDN is an ITU-R accepted standard, and ITU itself stands at the head of international communications standards – so, for example and contrary to some people’s belief, Kenwood’s NEXEDGE or Icom’s IDAS can be marketed and used in Europe with absolutely no legislative compliance problem. An ETSI standard provides a reference point, it’s not a ‘you can use it or you can’t’ directive. That’s a very important point to clarify, there’s a lot of misconceptions out there about that.”

Another misconception that Atkins is keen to flag concerns the extent of Kenwood’s ongoing support for dPMR. He says: “It is Kenwood’s aim to offer customers the right communication solution to meet their needs irrespective of the ‘flavour’ of the technology or protocol.

“Our R&D efforts are focused equally on FDMA and TDMA technologies and we will continue to do so for as long as there is a customer requirement to be met. Customers sometimes tell us, ‘Oh, I heard that you were pulling back from [dPMR]’ because we’ve introduced more DMR models recently. That’s simply not the case, it’s all about meeting customer needs and market demand, so we’ll keep developing equipment and solutions for both strands and treat them as equal.”

The quantity of proprietary features that many manufacturers layer on top of the DMR standard requires would-be users seeking to mix infrastructure and terminals from different manufacturers to look carefully at which features they need and which will work in such an environment.

How does dPMR fare in this regard? Hizzey says: “There’s not very much in the way of supplementary features that have been added on top of the dPMR standard because if there is some recognised supplementary feature, then we go out of our way to update the ETSI standard to include it. That’s one reason why the ETSI dPMR standard has evolved quite a lot, there’s been quite a few editions of the standard.”

He adds that when a supplementary service is created, the dPMR Association’s technical group usually agrees on how it should be implemented and this information is made available to the Association’s members, so they can implement it. “That’s something that doesn’t really exist within NXDN.”

While dPMR and DMR have both matured to the point of encroaching on each other’s original territory, perhaps this is to be welcomed as the competition it creates benefits end-users and the wider industry alike. It also highlights the fact that when you are looking to buy a new radio system or make the transition from digital, there is no harm in talking through all the options with your supplier.

Transitioning from analogue to dPMR“As all analogue systems are FDMA, there is no fundamental change of the technology [when migrating to dPMR],” says the dPMR Association’s Pete Hizzey.

“You could use your same frequency, licence and base station. You just need to change the controller that is providing the modulated signal. You can use a dual-mode controller that can operate five tone or MPT trunking and directly translate calls from one technology to the other.”

This allows dPMR terminals to be added one at a time – a soft transition from analogue. “If you try to do this with DMR, it’s TDMA, so a new base station is needed.” He adds that from a passive component perspective, there can be a higher cost with FDMA tech if the network uses multichannel equipment on a single antenna.

What to consider: the main dos and don’ts of buying FDMA radio systems

Do consider dPMR if you need a simple system for fast deployment.

Do remember that FDMA technologies offer a slight range advantage over analogue and TDMA – this may be particularly useful for (d)PMR466 users and when dealing with areas that are particularly difficult to cover.

Do note that dPMR’s ecosystem is similar to that of DMR in that ATEX, trunked and non-trunked versions and many functions including over-the-air programming and alarm integration are available.

Dosee if you can speak with an end-user who is using dPMR or NXDN with similar requirements to your own to get their perspective if you have any concerns.

Do involve the staff who will use the system in the decision-making process.

Don’t assume that FDMA technologies can’t be used in Europe – they can.

Don’t make the mistake of thinking that DMR is the only option when it comes to digital radio. Discuss with your dealer whether dPMR might be a better fit for your requirements.

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